Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through formulation of iron, nickel and cobalt-base superalloys, though such alloys alone are often inadequate to form components located in certain sections of a gas turbine engine, such as the turbine, combustor and augmentor. A common solution is to protect such components with an oxidation-resistant coating or a thermal barrier coating (TBC). Diffusion coatings, such as diffusion aluminides and platinum aluminides, and overlay coatings such as MCrAlY (where M is iron, cobalt and/or nickel) have been widely employed as environmental coatings and as bond coats for thermal barrier coatings on gas turbine engine components.
To be effective, thermal barrier coatings must have low thermal conductivity, strongly adhere to the article, and remain adherent throughout many heating and cooling cycles. The latter requirement is particularly demanding due to the different coefficients of thermal expansion between materials having low thermal conductivity and superalloy materials typically used to form turbine engine components. Thermal barrier coating systems (TBC systems) capable of satisfying the above requirements have generally required a bond coat, such as the above-noted diffusion and MCrAlY coatings. Ceramic layers formed of metal oxides such as zirconia (ZrO.sub.2) that is partially or fully stabilized by yttria (Y.sub.2 O.sub.3), magnesia (MgO) or other oxides, have been widely employed as materials for thermal barrier coatings. The ceramic layer is typically deposited by air plasma spraying (APS), low pressure plasma spraying (LPPS), or a physical vapor deposition (PVD) technique, such as electron beam physical vapor deposition (EBPVD) which yields a strain-tolerant columnar grain structure.
The aluminum content of the above-noted bond coat materials provides for the slow growth of a strong adherent continuous aluminum oxide layer (alumina scale) at elevated temperatures. This thermally grown oxide (TGO) protects the bond coat from oxidation and hot corrosion, and chemically bonds the ceramic layer to the bond coat. However, a thermal expansion mismatch exists between the metallic bond coat, alumina scale and ceramic layer, and tensile stresses generated by this mismatch gradually increase over time to the point where spallation can occur at the interface between the bond coat and alumina scale or the interface between the alumina scale and ceramic layer. Furthermore, though bond coat materials are particularly alloyed to be oxidation-resistant, the surface oxidation and interdiffusion (with the substrate) that occurs over time at elevated temperatures gradually depletes aluminum from the bond coat. Eventually, the level of aluminum within the bond coat can become sufficiently depleted to prevent further slow growth of the protective alumina scale and to allow for the more rapid growth of nonprotective oxides, the result of which again is spallation of the ceramic layer. In addition to depletion of aluminum, the ability of the bond coat to form the desired alumina scale can be hampered by the interdiffusion of certain elements between the superalloy and bond coat, such as during formation of a diffusion aluminide coating and during high temperature exposure.
From the above, it is apparent that the service life of a TBC system is dependent on the bond coat used to anchor the thermal insulating ceramic layer. Consequently, considerable research is directed to improved bond coats for TBC systems, including efforts directed toward the use of oxidation-resistant materials other than diffusion coatings and MCrAlY alloy coatings. One example is bond coats formed of an overlay (i.e., not a diffusion) of beta (.beta.) phase nickel aluminide (NiAl) intermetallic, whose composition is about 50 atomic percent (about 30 weight percent) aluminum, the balance being nickel. In the past, gas turbine engine components formed of NiAl intermetallic have been proposed. For example, advanced NiAl intermetallic alloys are reported in commonly-assigned U.S. Pat. Nos. 5,116,438, 5,116,691, 5,215,831 and 5,516,380 as suitable for forming gas turbine engine components. These patents are primarily concerned with including alloying additions to the NiAl intermetallic for the purpose of improving high temperature mechanical strength and low temperature ductility, neither of which has been previously of particular concern for TBC bond coats, since bond coats are deposited as sacrificial layers on the outer surfaces of components and do not contribute to the strength of the components.
NiAl intermetallic has also been proposed as an environmental coating in U.S. Pat. No. 4,610,736 to Barrett et al., in which additions of about 0.05 to 0.25 weight percent zirconium were shown to improve the cyclic oxidation resistance of the intermetallic, though whether such a coating could be used as a bond coat for a TBC system was not reported or evident. More recently, in commonly-assigned U.S. patent application Ser. No. 08/932,304 to Darolia, .beta.-phase nickel aluminide bond coat containing a limited amount of zirconium and/or other reactive elements such as hafnium, yttrium and cesium, is reported to promote the adhesion of a ceramic TBC layer to the extent that the service life of the resulting TBC system is drastically increased.
As with Barrett et al., Darolia teaches that additions of zirconium and other reactive elements serve to promote the oxidation resistance of .beta.-phase NiAl intermetallic. Darolia also teaches that minimizing the diffusion zone between a NiAl bond coat and its underlying substrate promotes the formation of an initial layer of essentially pure aluminum oxide, promotes the slow growth of the protective aluminum oxide layer during service, and reduces the formation of voluminous nonadherent oxides of substrate constituents that tend to diffuse into the bond coat, such as nickel, cobalt, chromium, titanium, tantalum, tungsten and molybdenum. As such, Darolia teaches that NiAl bond coats can perform extremely well as a bond coat for a TBC if properly alloyed and deposited to contain only NiAl intermetallic and zirconium or another reactive element. Notably, Darolia subscribes to the conventional wisdom that alloying additions of chromium, titanium and tantalum to a bond coat detrimentally encourage the growth of alumina scale, leading to reduced spallation life of a ceramic layer on the bond coat. Consequently, it is apparent that the alloying requirements of a NiAl coating differ from that of NiAl articles, such as those of previously-mentioned U.S. Pat. No. 5,516,380.
Even with the advancements of Darolia, there remains a considerable and continuous effort to further increase the service life of TBC systems by improving the spallation resistance of the thermal insulating layer.